1. Field of the Invention
Embodiments of the invention described herein pertain to the field of electric submersible pump (ESP) assemblies. More particularly, but not by way of limitation, one or more embodiments of the invention enable an electric submersible motor radial support bearing.
2. Description of the Related Art
Electric motors convert electrical energy into mechanical energy to produce linear force or torque and are used in many applications requiring mechanical power, such as pumps. In the case of an electric submersible pump (ESP), a multi-phase electric motor is typically used in conjunction with a centrifugal pump to lift fluid, such as oil or water, to the surface of a well. In particular, an ESP motor is typically a two-pole, three-phase, squirrel cage induction motor. The two-pole design conventionally runs at 3600 rpm synchronous speed at 60 Hz power. These electric motors include a stationary component known as a stator, and a rotating component known as the motor shaft. In ESP applications, the stator is energized by a power source located at the well surface and connected to the stator with an electric cable. The electricity flowing through the stator windings generates a magnetic field, and the motor shaft rotates in response to the magnetic field created in the energized stator. A rotor secured to the shaft rotates within the stator. The length of the shaft in ESP motors is long: typically between twenty-seven and thirty-two feet in length, although these shafts can potentially range anywhere from five to forty feet. To accommodate the length of the shaft in ESP applications, the rotor is divided into sections. The length of the wound stator determines the number of rotor sections.
Rotor sections are spaced apart from one another, and a stator bearing is located between each rotor section for maintaining the shaft in axial alignment. These radial support bearings are sometimes interchangeably referred to as “motor bearings”, “rotor bearings” or “stator bearings.” The stator bearings are non-rotating bearings that fit snuggly inside the stator bore. Conventional stator bearings are shaped like a hollow cylinder, with two parallel walls that extend on the inner diameter (ID) and outer diameter (OD) of the bearing. The outer wall is of constant radius and compressed against the stator bore. Typically, stator bearings do not rotate, but are permitted axial movement to accommodate thermal expansion during operation. Bearing sleeves are conventionally paired with the conventional stator bearings. Bearing sleeves are keyed to the shaft and rotate with the shaft inside the stator bearings. The stator bearings prevent the rotors from making contact with the stator bore. The motor is filled with high dielectric oil, which lubricates the bearings and transfers heat. Thus, these conventional bearings are hydrodynamic.
FIG. 1 illustrates a conventional stator bearing of the prior art. As shown in FIG. 1, conventional stator bearing 1000 is an annular cylinder. Conventional bearing outer diameter 1200 would be compacted against the stator bore with conventional anti-rotational tabs 1150 that lock conventional stator bearing 1000 to the stator of the ESP motor, although axial movement of conventional stator bearing 1000 may be permitted. Conventional bronze sleeve 1050 rotates inside conventional stator bearing 1000. A conventional key 1100 may lock conventional sleeve 1050 with the ESP motor shaft (not shown) that would extend through conventional sleeve 1050.
A plurality of stator bearings support the rotors on the common shaft. This arrangement is generally referred to as “a rotor stack”. The rotor stack is nested inside a stator that completes the inner workings of the motor. The motor shaft ultimately couples with a pump, or series of pumps, which drives the pump to produce fluid to the surface. In the case of an ESP motor, the motor is typically coupled from bottom to top, to a seal section (motor protector), intake and centrifugal pump. In some instances such as in gassy wells, a gas separator or charge pump may also be included in the assembly. Each section of the ESP assembly has a central shaft. The shafts are all attached, typically by spline, such that as the motor shaft rotates, all the shafts rotate.
ESPs have been in use for nearly a century and little has changed mechanically from the historical design. Most improvements have been in the nature of better wire insulation, lubrication and bearing materials. Historical design, however, does not support the modern trend toward directional drilling, which causes bends in downhole wells. In directional drilling, the drill bit may be realigned from a traditional vertical direction to a horizontal direction to reach larger pockets of oil or other desirable resources. To reach resources in a horizontal direction the degree of the bend of the hole should be large enough to allow the ESP equipment to pass through without any yielding of flanges, bolts or housings. However, in some cases the exact depth and true distance to the bend required is unknown in advance. Unexpectedly, the ESP equipment may be required to bend more than the conventional tolerance (not to be exceeded) of 10 degrees/100 feet. A bend greater than 10 degrees/100 feet will almost certainly destroy the stator bearings, which may cause motor failure. As the motor shaft is forced to bend, the bending causes side loading at the rotor producing up to 400 pounds of side load on the stator bearings, leading to bearing failure.
Other components of ESP assemblies suffer from the same problem, for example radial support bearings in the seal section, in the pump or in stages, during installation of the assembly or whilst the assembly is operating, depending on which section of the ESP assembly settles in the bend.
As is apparent from the above, currently available ESP assemblies are not engineered to support modern wells that contain bends. Therefore, there is a need for an improved electric submersible motor radial support bearing to increase the bend tolerance of ESP assemblies beyond 10°/100 feet.